# Aptitude - Odd Man Out and Series - Discussion

Discussion Forum : Odd Man Out and Series - Pick Wrong No. (Q.No. 5)
Directions to Solve
Find out the wrong number in the given sequence of numbers.

5.
1, 2, 6, 15, 31, 56, 91
31
91
56
15
Explanation:

1, 1 + 12 = 2, 2 + 22 = 6, 6 + 32 = 15, 15 + 42 = 31, 31 + 52 = 56, 56 + 62 = 92

Last number of given series must be 92 not 91.

Discussion:
22 comments Page 1 of 3.

Rajeswari Yarra said:   1 year ago
Easy to Understand. Thanks everyone for explaining the answer.

Utkarsh Kumar said:   6 years ago
2 is also a prime number. So 31 cannot be the answer even if we consider this question in terms of prime numbers.

This question is solved on the basis of perfect squares. Subtract each adjacent number and you will get a perfect square, except:

91-56 = 35.

35 is not a perfect square, so we make it 36 by adding 1 to 91. Thus the final series will be as follows:

1, 2, 6, 15, 31, 56, 92
where subtracting any two adjacent numbers will give a perfect square.

I hope this helps.
(8)

Jatin Dwivedi said:   7 years ago

2, 5, 15, 131, 530, 13257 find out wrong number of the series.

Fathima said:   8 years ago
31 is the only prime number.

Why not the answer is 31? Some one explain.

Arthy said:   8 years ago
2 - 1 = 1(odd),
6 - 2 = 4(even),
15 - 6 = 9(odd),
31 - 15 = 16(even),
56 - 31= 25(odd),
But 91 - 56 = 45 (odd) so that 91 is the wrong number here.
(3)

Manish said:   8 years ago
If we see prime numbers. Then 31 will be the answer. So which is the correct answer?

Sreeram said:   9 years ago
Why not 31? It's the only prime number.

Debasish said:   9 years ago
6-2 = 4 = 2*2.
15-6 = 9 = 3*3.
31-15 = 16 = 4*4.
56-31 = 25 = 5*5.
91-56 = 35! = 6*6.

Moni said:   9 years ago
What is the answer for sum?